The general kinetic description of two wave interaction in plasmas, obtained by perturbation expansion of the kinetic equation, is discussed along with various approximations. The general expression involves the full detail of the distribution function perturbations, f a and f b, of the two interacting waves, fa and fb, and is at present numerically intractable. Replacing the general expression by a truncated expansion in velocity space the low temperature kinetic (LTK) model is obtained. This model, which involves only the zeroth and first moments of fa and fb, is valid when thermal effects are small for the scattered wave but makes no assumptions about the interacting waves. Alternatively the LTK model may be derived from a truncated sequence of moments equations. The effect of truncating at different moments is discussed. To ensure that the limitations implicit in the truncated moments model apply only to the scattered wave and not to the interacting waves, it is found that the perturbation expansion must be carried out in variables which, like the electron flux, are linear in the distribution function, f. If the perturbation expansion is carried out in fluid velocity, which is non-linear in f, then the limitations of the truncated model apply also to the interacting waves. This is the principal cause of the limited range of applicability of the traditional cold fluid model of scattering. Finally the consequences of linear dependencies and cross correlations between fields and moments are considered.